Restricted earth fault Protection as the name suggests protects the restricted (fixed area) part of transformer from earth fault. This restricted part here implies the inside part of transformer only. That means that REF protection covers only the internal ground faults and for external ground faults separate earth fault relays are used. Unlike conventional earth fault protection that operates for any ground fault in the system, a REF scheme is designed to provide selective protection for internal faults in a transformer.
How internal ground fault occurs in a Transformer
Windings are covered with paper insulation , but if due to some reason or some fault the paper insulation breaks or there is insulation failure and the winding touches the transformer’s main body which is always grounded, then it will cause an internal ground fault.
Scheme for Restricted earth fault Protection
For detection of external ground or overcurrent faults, CT installed in the transformer bay is used and this CT is used to give current inputs to overcurrent & earth fault relay used for external faults. For internal faults, Current transformers are installed on both the input and output side of the transformer, these CTs accurately measure the current flowing into and out of the equipment and they are called bushing current transformer or turret C.Ts as they are located near bottom part of Hv and LV bushings.
Drawing shown below is of HV REF protection scheme and 87T is differential protection relay. Please observe in drawing below that the CT secondary of each phase of Hv side are connected together as shown. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. This NCT is used to measure current flow through the neutral of the transformer and the secondary is connected to restricted earth fault relay. The fault current flows through the neutral of the transformer only in unbalanced conditions i.e fault conditions. REF is basically differential earth fault protection, it actuates on the difference value of phase CT and NCT output currents.
Stabilizing resistor and Non linear Resistor
Kindly see in above drawing that a resistance is connected in series with the REF relay, it is called stabilizing resistor. Due to presence of this resistance, it is called High Impedance REF Protection. The stabilizing resistor guarantees that the relay will not trip during a through fault. For High Impedance REF , Phase CT and Neutral CT ratio should be same.
Non-linear resistors are resistors where the electric current flowing through it changes with the exchange in applied voltage or temperature and it does not change according to Ohm’s law. Metrosils (Non linear resistor) are used to limit the peak voltage developed by the current transformers under internal fault conditions, to a value below the insulation level of the current transformers, relay and interconnecting leads, which are normally able to withstand 3KV peak.
Connection drawing of Restricted earth fault Protection
Kindly observe following in the connection diagram of HV side Restricted earth fault Protection :
For all R,Y,B phase four core CT has been used in the connection shown below, two cores of these CTs have been used for Restricted earth fault Protection and unused cores are looped.
1U is connected with 1V and it is connected to 1W and then it is connected to 1N and from this 1N 1A171 ferrule is connected to Restricted earth fault relay .Similarly one more terminal is looped in this manner and another output to Restricted earth fault relay 1A172. In the drawing shown above, these REF terminals have been shown as 2A71 and 2A73.
Unused cores of U,V and W have been looped with each other in TB no.X7 &X8 ,X23&X24,X53&54.
In Auto-transformer only one Neutral is present in the HV side only, its connection for REF is done as :
Conclusion
REF protection in a transformer is done using turret CTs to detect and isolate earth fault in windings of the transformer , this protection is fast and confined to a fixed Zone i.e internal part of the transformer and it can isolate winding faults very quickly, it thus limits the damage of the transformer and a very important protective relay .